The present invention relates generally to laser communication systems, and more specifically to a laser transmitter which uses dichroic combination to combine the output beams of numerous diode lasers into a single diffraction-limited beam.
The laser beams of spaceborne laser communication systems must travel great distances (100 to 1,000 miles) in links involving space platforms. To achieve adequate signal margins for acquisition, track, and communication the beams must be focussed very sharply with minimum beam divergence so that the received beam appears to be its brightest (maximum irradiance). The optimum condition occurs when a full aperture beam is emitted from a near perfect (diffraction limited) telescopic optical system. If the source laser produces a near perfect beam, a diffraction limited telescope can project the laser beam into the brightest possible signal at the receiver.
One candidate being considered as a laser source is the laser diode. Laser diodes are small, efficient, and their low voltage drive requirements are compatible with the signal levels available from highly reliable solid state circuit components. In the design of a laser transmitter these features are important because they result in decreased system size and weight, reduced cooling and electrical requirements, and increased overall system reliability.
The favorable mechanical and electrical characteristics of laser diodes are complemented by a number of useful optical properties. The operating wavelength of a laser diode may be accurately tuned over a broad range by controlling either the diode temperature or drive current. Also, the output power of a laser diode may be directly modulated at frequencies up to several gigahertz by modulating the diode drive current. Separate modulator crystals and high voltage drive electronics are not required. Of particular importance is that laser diodes can be made to provide a single axial mode output. They are also highly polarized, a property which would be useful in combining, separating, or isolating signals.
The most serious opertional shortcoming of single mode laser diodes is the low output power levels which are now available--typically less than 50 milliwatts continuous wave (c.w.) signals. Undoubtedly, this limitation has in the past prevented the use of laser diodes in space related applications. Full utilization of the potential of laser diodes for free-space communications depends on the development of an effective method of increasing the power available in a near-diffraction limited beam. There are two approaches to this problem. One approach is to develop new diode designs which can provide higher single mode power while maintaining a long lifetime. Current research is directed at this approach and single mode diode power levels are increasing each year. It seems unlikely, however, that substantial increases beyond a factor of two or three will occur in the near-future due primarily to power density limitations.
A solution to the low output power drawback of diode lasers is suggested by the following U.S. Patents, the disclosures of which are incorporated by reference:
U.S. Pat. No. 3,808,428 issued to Barry et al; PA1 U.S. Pat. No. 3,835,414 issued to Ahearn; PA1 U.S. Pat. No. 3,920,983 issued to Schlafer et al PA1 U.S. Pat. No. 3,953,727 issued to d'Auria et al; and PA1 U.S. Pat. No. 4,406,003 issued to Eberly et al.
The d'Auria et al patent is pertinent for its disclosure of a transmitting and receiving system with independent communication channels in a single optical fiber. A plurality of laser diodes supply infrared rays of different wavelengths to respective ones of a series of selective mirrors. The mirrors are tilted at a 45 degree angle and downstream beams are directed to the exit sides of the mirrors where they are reflected and combined with the beams passing through from the upstream sides of the mirrors. In this way, the light radiation from the diodes is multiplexed to the single fiber.
Schlafer et al also discloses a multi-channel optical communications system which provides simultaneous optical communications over a plurality of different, non-interfering wavelengths. The device of this patent uses as a source a multiwavelength dye laser to provide a plurality of optical carriers in a single beam.
Eberly et al provide an optical communications system which uses pulses of different wavelength and/or polarization. Ahearn shows a gallium arsenide laser diode array.
Barry et al disclose a satellite laser communications system including a telescope.
While the above-cited references are instructive, the need remains to provide a solution to the low output power drawback of diode lasers so that they are suitable for use in spaceborne laser communications systems. The present invention is intended to satisfy that need.